Bomb sniffers sharpen nose

Portland, Ore. - A modified capacitive transimpedance amplifier promises to increase the sensitivity of airport explosives detectors one thousandfold, making it possible for handheld devices to sniff the air around baggage and passengers without delaying departures.

Despite the stepped-up federal emphasis on security since 9/11, it has been impossible to scan all airline passengers and their luggage for explosives. Today's technology is not sensitive enough. Machines now used in airports can only detect trace amounts of explosives from samples swabbed from suspicious articles, slowing the process.

University of Arizona professor Bonner Denton says the system he developed with other researchers will be smaller and more efficient. "Our entire analyzer system as well as detector electronics fits in a space 4 x 2 x 1.6 inches, and it can be made smaller than that," said Denton. "And it produces higher resolution as well as vastly improved sensitivity."

Denton collaborated with fellow University of Arizona scientist Roger Sperline as well as researchers Christopher Gresham and David Jones at Sandia National Laboratories in Albuquerque, N.M.

Sandia is slated to use the chip in a handheld version of the explosive detector that will replace bomb-sniffing dogs. Sandia calls its device a "microhound."

Denton's variation on the capacitive transimpedance amplifier improves the readout circuitry in current ion-mobility spectrometers. These devices, which are the size of a microwave oven, are already being used at airports to detect traces of explosives from swabs run over suspicious bags. Denton's improvement will enable ion-mobility spectrometers to sniff the air around people who have handled explosives as well as their baggage.

"This is the first radical change in ion-mobility spectrometers since the 1930s," Denton said. He's negotiating with a manufacturer of airport ion-mobility spectrometers, hoping to replace those devices with his smaller, more-sensitive system. "Hopefully, we can work out a deal where we can spin this technology off for them to start producing smaller scanners that make all of us safer at airports," said Denton.

A capacitive transimpedance amplifier is an electronic circuit that uses a high-gain op amp with a capacitor on its input as a charge integrator. The output of the circuit increases as more charge is stored on the capacitor. What makes Denton's implementation better is the diminutive size of the capacitor, which stores a 10-femtofarad charge.

"My research group was the first to utilize charge-coupled devices and charge-injection devices for optical spectroscopy. Now virtually all areas of low-level-light spectroscopy use this technology. We then moved into the world of infrared detector technology, but it turns out infrared detectors have to be made somewhat differently, because you have to use very exotic materials that can absorb infrared photons and convert them into electron-hole pairs or charge. To detect this charge, a variety of electrical engineers have put a lot of effort into developing capacitive transimpedance amplifier technology," said Denton. "My part was to realize that instead of using the exotic infrared material, all one had to do was put an electrode on top of this and it would be an exceedingly good detector of charge wherever it came from.

"We want to see this technology spun into commercial devices," Denton said. "We want to ensure that no matter how careful the bad guy was in preparing his explosive-no matter how much hand-washing and cleaning up, and no matter how well he sealed it up-we are still going to catch him."

Each of Denton's detector chips has four capacitive transimpedance amplifiers on a 2 x 2-mm die, which was fabricated in 0.1-micron CMOS at the Metal-Oxide Semiconductor Implementation Service (Mosis). The research was funded by the Department of Energy.